The present invention relates to a method for forming an electrostatic image and, more particularly, to a method for forming an electrostatic image of an original having black and colored portions, wherein the black portion is selectively eliminated from the electrostatic image. The present invention also relates to an apparatus therefor.
In the formation of a color copy from an original having both black and colored portions by an electrophotographic process, it is generally desirable for the black portion to be reproduced with black toner alone. The reason for this preference is that it is practically impossible to reproduce true black by the mixture of three different toners representing the primary colors. Moreover, where the black image is reproduced by superposingly applying black toner on color toner, it is difficult to effect complete registration or coincidence of the black image with the color image. This is actually the case when a color original with a black portion to be copied contains a line, a letter, a character, or the like.
According to known methods for the formation of a color copy of an original having both black and colored portions by an electrophotographic process, an electrostatic image corresponding only to the black portion of the original can readily be obtained by imagewise projecting a light image of the original through three primary color filters onto a uniformly charged photoconductive member. However, it is difficult to form an electrostatic image of the original in which only the black image portion is eliminated, because the black image portion absorbs all of the visible rays; as a consequence, such image formation has heretofore been possible only by means of a complex process utilizing, for example, a negative image of the black portion.
In view of this current state of the art, the primary object of the present invention is to provide a method for forming an electrostatic image of an original having both black and color image portions, wherein the electrostatic image corresponding to the black portion of the original can be easily and effectively eliminated.
Another object of the present invention is to provide an electrophotographic apparatus suitable for producing a color copy of a dichromatic or multi-chromatic original containing a black portion therein. According to the present invention, since the electrostatic image corresponding to the color portion and the black portion of the original can be separately and independently obtained, it is possible for the electrostatic images of these portions to be developed with different toners of different colors independently but successively so that a clear copy of the original having black and colored portions can be obtained by means of a relatively simple electrophotographic process.
For example, a two-color or multicolor copy image in which the black image portion is clearly reproduced can be obtained by combining a process for developing an electrostatic image using toner of the same color as that of the color image portion of an original, or repeating the same for each of the multicolor image portions of the original, with a process for developing, with black toner, an electrostatic image corresponding to the black image portions of the original.
Thus, the present invention more specifically relates to a method for the formation of an electrostatic image of an original having black and color portions, wherein the black portion is selectively eliminated from the electrostatic image, and comprising:
(a) forming a first electrostatic image corresponding to the black and color portions of the original on a first recording material capable of bearing an electrostatic charge thereon;
(b) forming a second electrostatic image corresponding to the black portion of the original on a second recording material capable of bearing an electrostatic charge thereon, the second electrostatic image being a reflected image with respect to the first electrostatic image; and
(c) eliminating the electrostatic image corresponding to the black portion of the first electrostatic image either by superposing the first recording material on the second recording material, or by placing the first recording material in close proximity to the second recording material, so that the electrostatic image of the black portion of both the first and second recording materials face each other, and by impressing on the first electrostatic image an electric charge of a polarity opposite that of the first electrostatic image either by direct use of the second electrostatic image or by use of the second electrostatic image as a control medium.
The invention further relates to an apparatus for carrying out the foregoing method.
According to one of the preferred embodiments of the present invention, the method comprises:
(a1) forming a first electrostatic image corresponding to the black and color portions of the original on a first recording material capable of bearing an electrostatic charge thereon;
(b1) forming a second electrostatic image of the same charge polarity as that of the first electrostatic image on a second recording material capable of bearing an electrostatic charge thereon, the second recording material being a screen member which controls a flow of electrically charged particles only in the region corresponding to the black portion of the original, and the second electrostatic image being a reflected image with respect to the first electrostatic image; and
(c1) eliminating the electrostatic charge in the region corresponding to the black portion of the first electrostatic image either by superposing the first recording material on the second recording material, or by placing the first recording material in close proximity to the second recording material, so that the electrostatic image of the black portion of both the first and second recording materials face each other, and by projecting a flow of charged particles having an opposite polarity with respect to that of the first electrostatic image onto the first electrostatic image through the second recording material.
According to another preferred embodiment of the present invention, the method comprises:
(a2) forming a first electrostatic image corresponding to the black and color portions of the original on a first recording material capable of bearing an electrostatic charge thereon;
(b2) forming on a second recording material an electrostatic image corresponding to the black portion of the original and having an opposite polarity with respect to the first electrostatic image, the second electrostatic image being a reflected image with respect to the first electrostatic image; and
(c2) eliminating the electrostatic image corresponding to the black portion of the first electrostatic image either by superposing the first recording material on the second recording material, or by placing the first recording material in close proximity to the second recording material, so that the electrostatic image of the black portion of both the first and second recording materials face each other, and by effecting on the first electrostatic image an electric charge of the second electrostatic image.
In practicing the present invention, the first recording material may be any sheet material capable of bearing an electrostatic charge on its surface. Thus, it may comprise any conventional photoconductive plate having a photoconductive layer on an electrically conductive support--including a so-called screen type photoconductive plate which comprises an electrically conductive substrate having a plurality of holes therethrough and a photoconductive layer on one surface of the conductive substrate--or alternatively it may comprise a sheet material consisting of an insulating substance onto which an electrostatic image can be transferred from the photoconductive plate.
The second recording material may be formed the same as the first recording material. It is preferred, however, that the aforementioned screen-type photoconductive plate be employed for the second material, especially when the processes described above as (b1) and (c1) are employed in the image formation procedure. Where the processes described as (b2) and (c2) are employed, on the other hand, any of those plate types or structures which can be used for the first recording material may be utilized. From a practical point of view, a photoconductive plate may advantageously be employed.